Turbidimeters measure the turbidity (cloudiness, haziness) of a solution, e.g., water, by viewing light that passes through the solution and determining how much light is transmitted or reflected (depending on the detection technique of the turbidimeter). For example, the Hach Company TU5 Series turbidimeters measure turbidity by directing a laser into a sample to scatter off suspended particles in solution. The light that is scattered at a 90° angle from the incident beam is reflected through a conical mirror in a 360° ring around the sample before it is captured by a detector.
The amount of light scattered is proportional to the turbidity of the sample. If the turbidity of the sample is negligible, little light will be scattered and detected by the photocell, and the turbidity reading will be low. High turbidity, on the other hand, will cause a high level of light scattering and result in a high reading.
Turbidimeters need to be calibrated. This is accomplished using a standard solution of known turbidity. For example, a standard containing the formazine (formazin) polymer is commonly used. The formazine polymer is dissolved into solution to form a standard solution of known turbidity unit values (i.e., having a known amount of nephelometric turbidity units (NTUs)), creating a formazin stock solution. For example, 5 g/L hydrazine sulfate is mixed with 50 g/L hexamethylenetetramine in ultrapure water, and developed over 24 hours at 25° C. to produce a suspension with 4000NTU (formazin stock solution). This formazin stock solution may then be diluted to an appropriate value to act as a standard for a given detection range. Information on standard units and solutions commonly used in this area is found in the international standard (ISO 7027).